Interaction of non-allelic genes
Many traits depend not on one but on several non-allelic genes (different genes on different pairs of chromosomes); they obey Mendel’s law of independent inheritance but the phenotype ratio changes. In complementary action the trait appears only when both dominant genes are present together: in sweet pea AAbb (white) × aaBB (white) → F1 AaBb red; F2 has 9 red : 7 white; in the chicken comb A_B_ is walnut, A_bb rose, aaB_ pea and aabb single (9:3:3:1). In epistasis the dominant (or recessive) state of one gene suppresses the action of another, non-allelic gene: in chickens the inhibitor gene I stops the colour gene C, so I_ plumage is white; IiCc × IiCc gives in F2 13 white : 3 coloured. F2 ratios in epistasis: 13:3 or 12:3:1 for dominant epistasis, 9:3:4 for recessive epistasis; in complementary action 9:7, 9:6:1, 9:3:4 and 9:3:3:1. The sum of all these ratios is always 16.
“Ratio hunt”. Write the ratios 9:3:3:1, 9:7, 13:3, 9:3:4 and the kinds of action (ordinary dihybrid, complementary, dominant epistasis, recessive epistasis) on cards; match them in pairs. Check that each ratio sums to 16.